Aluminum is the third most used material employed to fabricate silicon solid state components (the other two being silicon and silicon dioxide). Aluminum is primarily utilized in these applications in thin-film form, and it functions as a material which interconnects the structures of the device formed in the silicon substrate. It has emerged as the most important material for such applications because of its low resistivity and its compatibility with the other two substances. For example, aluminum thin films adhere well to silicon dioxide. During the thermal step that sinters the metal silicon contacts, the aluminum atop the silicon dioxide forms a thin layer of aluminum oxide at the aluminum/silicon oxide interface, and this promotes good adhesion. The material properties of aluminum films which are of most interest for silicon device fabrication are its melting point of about 660.degree. C., the formation of an Al/Si eutectic at about 577.degree. C., its density of 2.70 g/cm.sup.3 and its resistivity of 2.7 micro ohms-cm. Aluminum alloys are also often utilized in microelectronic applications, including aluminum with 1 weight percent silicon; aluminum with 2 weight percent copper; aluminum with 4 weight percent copper; aluminum with 1 weight percent silicon and 2 weight percent copper; and aluminum with 1.2 weight percent silicon and 0.15 weight percent titanium. Note that the relative low values of the melting point of aluminum and the Al/Si eutectic temperature, restrict the maximum value of subsequent processing temperatures once the Al film has been deposited.
Aluminum thin-films are deposited as polycrystalline materials, usually in the 0.5-1.5 micron thickness range. Early microelectronic devices utilized evaporation to deposit pure aluminum, Al--Si or Al--Cu alloys, but the stringent alloy-composition requirements of VLSI (and other limitations of evaporation) have made sputtering the dominant PVD technology for aluminum films. The development of magnetron sputtering, which allows aluminum to be deposited at deposition rates up to about 1 micron/min, further decreases the use of evaporated aluminum film deposition. Aluminum alloys are used more frequently than pure aluminum because they possess enhanced properties with certain interconnect requirements, including superior contact formation characteristics and better resistance to electromigration.
Aluminum readily forms a thin native oxide (Al.sub.2 O.sub.3) on its surface upon its exposure to oxygen, even at room temperature. The presence of this oxide can affect the contact resistance when another metal layer is deposited on the aluminum, and can inhibit both the sputtering of an aluminum target and the etching of aluminum films. Aluminum thin films can also suffer corrosion problems as a result of some fabrication processes. For example, if phosphorus-doped silicon dioxide is deposited onto an aluminum film, phosphoric acid can be formed if moisture is absorbed by the glass. This acid will attack aluminum and cause corrosion. In addition, dry etching of aluminum may leave chlorine residues on the aluminum surfaces. Exposure to ambient moisture can lead to the formation of HCl. If copper is present as an alloy in the aluminum film, severe corrosion can occur (i.e. the CuAl.sub.2 compound and the aluminum form microelectrodes of a battery with HCl acting as the electrolyte).
The etching of aluminum and aluminum alloy films is a very important step in the fabrication of integrated circuits. The device density, on many of the most advanced circuits, is limited by the area occupied by the interconnect paths. Anisotropic etching of the metal layers permits the use of small minimum pitches (i.e., the pitch is the sum of the dimensions of a metal line and the space between lines), which increases the interconnect capability. Smaller metal pitches create greater vulnerability to corrosion. Furthermore, the isotropic nature of aluminum wet etching processes renders them inadequate for VLSI applications and therefore there is a need for a directional dry etching process.
Reactive etching of aluminum films using chlorides is commonly practiced. It has been determined that a freshly exposed aluminum surface, uncovered by aluminum oxide will react spontaneously with Cl or molecular Cl.sub.2 to form AlCl.sub.3, even in the absence of a plasma. If, however, the surface of the aluminum is covered with a thin layer of aluminum oxide, it will not react with Cl or Cl.sub.2. Thus, the etching of aluminum films is a 2-step process, involving removal of the native oxide layer, and etching of the aluminum film.
The successful removal of the native oxide is one of the most important steps in achieving an effective aluminum etching process. Removal of aluminum oxide is far more difficult than the etching of pure aluminum, and the thickness of this oxide can vary from run-to-run depending on several factors. Thus, an aluminum etch cycle is observed to begin with an initiation period, during which the native oxide and the moisture from the chamber is slowly removed. The removal can be accomplished by sputtering with energetic ions, a condition that can be established in reactive ion etching systems, or by chemical reduction. The chemical reduction of aluminum oxide requires the availability of oxide reducing species. Again, chlorides are the materials of choice.
Water vapor must be excluded from the etch chamber in order to achieve reproducible aluminum etch processes. If an etch chamber is exposed to ambient after an etch run, moisture can be absorbed on the chamber walls. This condition becomes more severe if the etch product of aluminum, AlCl.sub.3, is allowed to deposit on the chamber walls. Such deposition occurs on surfaces maintained at room temperature. AlCl.sub.3 is very hygroscopic, and thus absorbs considerable moisture on exposure to the atmosphere. This moisture may be deabsorbed after the plasma is struck. The deposition of AlCl.sub.3 also has other deleterious effects on the etch process and several techniques are employed to manage AlCl.sub.3. Water vapor in the etch chamber is most effectively reduced by load lock chamber designs.
Small quantities of other materials are added to aluminum to improve some of its properties. That is, 1-2% silicon is often added to prevent the aluminum from spiking through shallow junctions, and 2-4% copper or 0.1-0.5 weight percent titanium are added to enhance the electromigration resistance. Copper forms an etch product with chlorine, CuCl, that is relatively non-volatile. Thus, copper and chloride containing residues often remain after these alloy films have been dry etched. This makes aluminum-copper alloys more difficult to etch in chlorine plasmas. The degree of difficulty increases with increase in copper concentration, and a 4% copper-containing film poses quite a formidable dry etching challenge. Methods must be employed to promote CuCl deabsorption.
Another problem with etching aluminum is that of post-etch corrosion. This effect arises from the hydrolysis of the chlorine or chlorine-containing residues (mostly AlCl.sub.3) which remain on the film sidewalls, substrate or resist after etch. Upon absorbing moisture, these residues form HCl which corrodes the aluminum. The reaction of HCl and aluminum produces more AlCl.sub.3 and thus as long as moisture is available, corrosion will continue. The problem is even more severe in the Al--Cu alloys since Al--Cu compounds formed in the film (primarily CuAl.sub.2) create a galvanic couple with the aluminum and this drives the corrosion even more rapidly than in pure aluminum films. Consequently, the residual chlorine and ambient moisture level necessary to induce corrosion are much lower when copper is present in the aluminum film.
Various techniques have been suggested to deal with post-etch corrosion, all involving the removal of the chlorinated species. These include: (a) removing the wafers from the chamber and rinsing in cold deionized water; (b) plasma ashing of the resist in an oxygen plasma before removal from the etched chamber, which removes the chlorine present in the resist, and restores the passivating aluminum oxide layer; and (c) exposing the aluminum to a fluoride-containing plasma before removal from the chamber which causes the chemically absorbed chlorine to be replaced with fluorine thereby passivating the aluminum by the formation of nonhygroscopic AlF.sub.3. Use of CHF.sub.3 as a source of fluorine is thought to deposit a column of film over the aluminum, thus sealing the surface and preventing moisture from penetrating to the chlorine residues. However, long term reliability studies on the effectiveness of these treatments, especially on circuits and plastic packages, have yet to be reported.
It would be advantageous if the corrosion of metal lines in semiconductor devices could be inhibited or altogether prohibited. Corrosion is believed to have the following causes:
1. Transport of moisture and such contaminants as Cl through the passivation layer, and subsequent reaction of these with the metal lines. The moisture and contaminants may exist in the package material itself or may arrive through cracks in the package. PA1 2. Leaching of phosphorus from phosphorus-doped SiO.sub.2 intermetal or passivation dielectric layers, followed by reaction of the phosphorus with absorbed moisture to form phosphoric acid which then attacks the aluminum lines. PA1 3. Residual Cl which remains on the surface following a Cl-based dry-etch process, reacts with moisture to form HCl, which then attacks the aluminum.
As stated, the standard way of dealing with the later problem has been to expose the wafer to a short, in situ CHF.sub.3 or CF.sub.4 etch step after the aluminum has been patterned and the resist has been dry etched. A recently reported alternative technique uses a reactor that contains an integrated spin-rinse system in the exit load lock, which sequentially dispenses an organic solvent and water onto the wafer. This technique is believed to remove the carbonaceous chlorine-containing polymer that remains on the wafer following the etch process.
It would be advantageous if an inherently corrosion resistant metal film layer could be found.